Automator keeps using numbers in exponential form
A simple and fast way requires to install a scripting edition Satimage.osax (direct d/l) and works with regex. The certificate of the pkg sadly expired!

on run {input, parameters}
    set text1 to change "^0+" into "" in (the clipboard as string) with regexp
    do shell script "open https://api.whatsapp.com/send?phone=971" &amp; text1
    return input
end run


^0+ strips off leading zeros!



A second one with sed but no additional installs:

on run {input, parameters}
    set text1 to do shell script "echo " &amp; quoted form of (the clipboard as string) &amp; " | sed 's/^0*//'"
    do shell script "open https://api.whatsapp.com/send?phone=971" &amp; text1
    return input
end run



quoted form of (the clipboard as string): '0501234567'
do shell script "echo " &amp; '0501234567' &amp; " | sed 's/^0*//'": run shell command echo '0501234567' | sed 's/^0*//' in an Apple script
echo '0501234567' | sed 's/^0*//': send the output of echo to the stream editor sed and do something with it
^0*: regular expression: ^ = start of line * = quantifier- matches between zero and unlimited times, as many times as possible
's/^0*//': 's/reg_ex/replacement/': substitute the replacement string for the first instance of the regular expression in the pattern space. This means: replace as many leading zeros as possible with the replacement string (=NIL/nothing) = strip off leading zeros
set text1 to ...: $text1=501234567
do shell script "open https://api.whatsapp.com/send?phone=971" &amp; text1: open https://api.whatsapp.com/send?phone=971501234567




Both tested in 10.11.6 (El Capitan) only.