Yeastar S20

Yeastar S20 is a modular VoIP PBX for small-to-midsize businesses that lets you choose what kinds of port work for your deployment. The S20 has two on-board module slots and can support the following modules:
S2 Module: 2 FXS (analog)
O2 Module: 2 FXO (analog)
SO Module: 1 FXS 1FXO (analog)
B2 Module: 2 BRI (digital)
GSM Module: 1 GSM (cellular)
3G Module: 1 UMTS (cellular)
Note that the S20 only has one antenna jack, so only one of the GSM or 3G Modules can be integrated.
Yeastar S20 IP-PBX handles 20 users with a maximum of 10 concurrent calls, giving you full telephony functionality including call routing and transfer, fax to email, conferencing, queuing, and much more. Using the Linkus Mobile Client app for Android and iOS, you can even make mobile phones part of the office phone system. Managing the phone system is simple using the graphical user interface of the online Configuration Panel, which allows you to access the phone system securely from anywhere with internet access. The S20 supports the TLS and SRTP transport protocols, which secure your communications. This compact IP-PBX can be desk or wall mounted.