## 3 Related Work  
To the best of our knowledge, PoT [8] is the first proposition in the literature of a general-purpose, open-source, and multi-tier IoT framework based on the integration between phones (PSTN and VoIP) and IoT technologies. Previous research has targeted specific applications and employed limited phone functionalities in third party systems as their proposed IoT systems. For example, [PERSON] et al. [10] proposed to enable IoT devices with a SIP-based communication protocol called \"CoSIP\", a constrained version of the Session Initiation Protocol (SIP) used in VoIP communication, to provide constrained smart objects with a lightweight and standardized protocol to bring them IP connectivity. Since then, other research work has been intermittently emerging to provide IoT solutions with the help of some phone functionalities. [PERSON] et al. [11], for example, proposed a SIP-based IoT gateway for healthcare applications. [PERSON] et al. [12] proposed a smart postbox system that enables delivery inspection and confirmation using VoIP technologies. Other IoT and VoIP integration applications are presented in [13; 14; 15]. Basically, these applications did not embed an IP-PBX server on the IoT gateway itself. However, they depend on utilizing a SIP client or an external IP-PBX server to provide phone functionalities to their proposed systems.  
Businesses typically mitigate the impact of missed calls through voicemail. Voicemail [16] is a mature feature that comes off-the-shelf with the most unified communication (UC) solutions. Voicemail stores voice messages that the caller can optionally leave to be retrieved later by the called person. The recipient can retrieve voice messages on a phone, desktop, or email account based on the UC platform utilized. However, statistics reveal that 80% of phone calls to businesses go to voicemail, and the average voicemail response rate is less than 5% [17]. [PERSON] et al. [18] proposed to send an SMS notification to the person missing a call on their desk phone. [PERSON] et al. [19] interfaced with an Asterisk system and looked for a series of SIP messages that indicate missed calls. They then extracted the called phone number from each message, queried a database for the corresponding mobile number of the called person, and send a text message if a result was found. IBM holds a patent [20] that utilizes SMS notification to a remote phone device when a missed call occurs. The approaches mentioned above, however, are somewhat similar. They do not provide a solution to maximize availability, decrease missed call occurrences, and help reduce their corresponding drawbacks. Nevertheless, they provide means to facilitate the retrieval of the missed calls list. Organizations can do better by eliminating the utilization of voicemails in their firms and providing better availability of their employees to mitigate the impact of missed calls in the first place. Moreover, call centers have revealed that a voicemail may be seen as a negative encounter for customers and subtracts from their satisfaction. Customers usually need instant answers to their inquiries and are not willing to wait for hours for someone to call back and help [21].