3, GSM and SMS Gateway with API

I am going to split this up so hopefully you can understand where I am coming from.

Background
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A client has a SMS system that was sending out via a Nokia 6130i connected to a GSM SMS gateway software package. The package communicated over COMM port to the phone and sent message over UK O2 network (on GSM). The software has an API which allows messages to be automatically included and sent via their in-house systems when certain triggers are called.

Problem
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Customer was advised to switch to different network (3 UK) to save a small fortune on SMS and call costs. A new phone was provided as part of the setup (as far as I understand the 3 network does not support non 3G/UMTS devices) the phone and COMM ports had been setup before I got on site and system was not working.

Discussion
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Hopefully someone can point me in the right direction, but I fear the software that communicates over a the COMM port will use a GSM specific protocol for sending out SMS. If this is true, will this be a restriction on the network or the phone? Any help or direction will be greatly appreciated, many thanks