The tones are played by the entry system and you likely have no control over the sound. However, one of the below options may help:
1. Set up a speed dial in the OBi and program the entry system to dial e.g. 2#.
2. Modify the digit map for the phone port; the entry system would dial a single digit, probably # or 0, to make the call.
3. If the entry system can be set to dial no digits at all, set up the OBi as hot line (if dedicated) or warm line.
4. If the entry system can do pulse dialing, set it to dial "1". The OBi doesn't actually understand pulse dialing, but it can be set to ignore the pulse, and connect the call via a hot line or warm line setup.