You can sign up to one of these open source PBX packages based on Asterisk:

Elastix has become an increasingly popular PBX software distribution aimed at unified communications including SugerCRM and vTiger CRM software, with an optional Call Center software module.		What was once known as Asterisk@Home has since been renamed Trixbox CE – the fully open source variant of Fonality’s commercial Trixbox offer. Trixbox CE can also be used with the powerful desktop telephony application HUDlite.
Our hosted Elastix starter package:	PBX E-Starter	Our hosted trixbox starter package:	PBX t-Starter
Number of users/connections	Unlimited	Number of users/connections	Unlimited
Average usage	1-8 employees	Average usage	1-8 employees
Maximum number concurrent calls	4	Maximum number concurrent calls	4
UK/USA DID number	1	UK/USA DID number	1
Included minutes outgoing calls	300 per month	Included minutes outgoing calls	300 per month
Guaranteed minimum RAM	256 MB	Guaranteed minimum RAM	256 MB
Diskspace	5 GB	Diskspace	5 GB
Patches & Backups	Included	Patches & Backups	Included
Setup costs	None	Setup costs	None
Monthly costs	Only € 34.95	Monthly costs	Only € 34.95

Hosted VoIP Solutions are quickly becoming one of the fastest ways for small and medium businesses to substantially cut costs without sacrificing quality. At PBXcomplete, we are here to make that process even easier, offering package VoIP solutions to help you further streamline your business.

Each hosted PBX comes pre-installed and ready-to-configure  on your own virtual private server, ensuring that your VoIP solution is designed to meet your company’s specific requirements. After you complete your order, we will send you the log in details within 24 hours so that you can access your hosted PBX. As soon as you log in, you can start adding extensions and users, and structure your VoIP phone system to meet your current needs. Our hosted PBX packages are set up with at least 1 UK or US VoIP phone number for you to receive calls immediately. Outgoing calls with one of the lowest call tariffs available, can also be made directly with your monthly balance.

There is no worrying about the maintenance of your installation, as PBXcomplete takes care of all of the necessary patches and updates of your virtual PBX server and PBX software. And since every night a backup is made of your server, you never have to worry about losing any of your changes.

PBXcomplete is your complete VoIP PBX provider, offering you everything you need to help streamline your business.