What version of IOS are you running on your 3745? If you are running 12.3(4)T or earlier, you will not be able to use this command.
An easy way to figure out what is available is to do:
VG_C3745(conf-voi-serv)#allow-connections ?
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Cisco IOS Release 12.3(4)T, Cisco IOS Release 12.3, and Earlier Releases
This command is used to allow connections between specific types of endpoints in a Cisco multiservice IP-to-IP gateway. The command is enabled by default and cannot be changed. Connections to or from POTS endpoints are not allowed. Only H.323-to-H.323 connections are allowed.
Cisco IOS Release 12.3(7)T and Later Releases
This command is used with Cisco Unified Communications Manager Express 3.1 or later systems and with the Cisco Multiservice IP-to-IP Gateway feature. In Cisco Unified Communications Manager Express, the allow-connections command enables the VoIP-to-VoIP connections used for hairpin call routing or routing to an H.450 tandem gateway.
Source: http://www.cisco.com/en/US/docs/ios/voice/command/reference/vr_a1.html#wp1632708
HTH
-Adam
