Google and Twitter said they were reactivating the system which was used in 2011 when the Internet was shut down in Egypt by authorities for several days.
Google's David Torres acknowledged that many Syrians may not be able to use the service because telecom networks in the war-ravaged country are also functioning poorly.
"But those who might be lucky enough to have a voice connection can still use Speak2Tweet by simply leaving a voicemail," he said.
Those able to dial out can leave messages on several phone numbers: +90 212 339 1447 or +30 21 1 198 2716 or +39 06 62207294 or +1 650 419 4196, "and the service will tweet the message," Torres said.
"No Internet connection is required, and people can listen to the messages by dialing the same phone numbers or going to twitter.com/speak2tweet."
AFP correspondents noted that internet and telephone communications, including mobile phones, were cut in the capital.
On Thursday, activists accused the regime of preparing a "massacre" when the telephone lines and Internet first went down, while the authorities explained the cut was due to "maintenance" work.
Washington branded it as a desperate move on part of the regime.