When installing SIP Trunks and VoIP, it all comes down to planning. In brief, a traditional ISDN channel (which has a bandwidth of 64Kbits), connects your telephone system to the Public Switched Telephone Network (PSTN). It allows you to make voice calls but does not allow you to send data. SIP Trunking connects your PBX phone system to the PSTN with an IP network instead of ISDN.
Voice over Internet Protocol (VoIP) is packet-switched telephony across the internet. The packets are often compressed and encoded and may take different paths to the same destination.
Internet Connection: To get the most out of VoIP, you will need an internet connection that offers enough performance to accommodate an appropriate call volume for your company.
Super-fast cable modem or DSL: While these are great for things like browsing the Web or if you work from home, they are often not suitable for VoIP because they provide lots of downstream bandwith but little upstream (away from your network) bandwith.
For VoIP, upstream bandwidth is every bit as important as downstream, particularly if you plan to use more than one phone.
So if you have a large company you would need to consider a high-speed internet connection and make sure you get the Internet connection and VoIP service from a single provider and check out the Service Level Agreement (SLA).
Compare SIP with ISDN and you can see the number of extra features you can have:
In summary, the key to success with SIP comes down to proper planning.