'1' first is country code for US and a domestic call. Generally, dialing '011' then the country code and number is the dialing format for international calls from the US. I don't have phone service thru my Comcast account so I don't know if international dialing is different.
Dialing just the country code first might confuse the local routing switches, thinking the first three digits are an area code (dialing 1+ for domestic LD is not mandatory). The '011' triggers the switches to route the call internationally.
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